The present invention relates to an apparatus and a method for estimating the amount of exhaust gas recirculation that is present in an internal combustion engine, or an internal EGR amount of an internal combustion engine.
Some of combusted gas in a typical internal combustion engine remains in a cylinder. The remaining combusted gas is referred to as internal EGR. Since internal EGR affects the combustion state and the emission of the engine, the amount of internal EGR is estimated and, based on the estimation, the amount of intake air, the ignition timing, the amount of injected fuel, and the fuel injection timing are adjusted. In an internal combustion engine having a variable valve actuation mechanism, the amount of exhaust gas that returns to the cylinder from the exhaust system is changed according to changes in, for example, the amount of valve overlap. The amount of internal EGR is changed accordingly. Valve overlap refers to a state where an intake valve and an exhaust valve are open at the same time. A valve overlap amount refers to a period in which valve overlap occurs in terms of an angle of a crankshaft, which is an output shaft of an internal combustion engine.
Accordingly, apparatuses for estimating the amount of internal EGR have been proposed. Japanese Laid-Open Patent Publication No. 2001-221105 discloses such an apparatus. The apparatus of the publication estimates the amount of internal EGR in consideration of changes in the amount of valve overlap based on the engine speed, the amount of valve overlap, the closing timing of the exhaust valves, and the intake pressure. Specifically, based on the closing timing of the exhaust valve and the engine speed, the apparatus first computes the amount of internal EGR during a period in which there is no valve overlap. Thereafter, the apparatus corrects the computed value based on the period of the valve overlap and the intake pressure, thereby estimating the amount of internal EGR.
The logic used in the above apparatus for estimating an internal EGR amount is not based on the physical behavior of combusted gas in actual internal combustion engines. This complicates the estimation logic. For example, equations and conditions for estimation must be changed according to various parameters such as the state of valve overlap and the closing timing of the exhaust valve. The estimation logic is particularly complicated when applied to an engine having a variable valve actuation mechanism of a high flexibility of control, which, for example, independently controls the valve timing of each of the intake and exhaust valves. If the estimation logic is complicated, the number of necessary parameters and the number of corrections based on the parameters are increased. Errors contained in the parameters and the corrections are accumulated and, as a result, degrade the accuracy of the estimation of the internal EGR amount.